Step-down coaxial microwave ablation applicators and methods for manufacturing same

ABSTRACT

Microwave ablation applicators and methods for manufacturing the microwave ablation applicators are disclosed. A microwave ablation applicator includes a feed-line segment, a step-down segment, and a radiator base segment. The feed-line segment includes a first inner conductor, a first dielectric disposed on the first inner conductor, and a first outer conductor disposed on the first dielectric. The step-down segment includes a second inner conductor, a second dielectric disposed on the second inner conductor, and a second outer conductor disposed on the second dielectric. The radiator base segment includes a third inner conductor disposed on the third inner conductor, a third outer conductor disposed on the proximal end of the third dielectric so as to form a feed gap at a distal end of the radiator base segment, a balun dielectric disposed on the third outer conductor, and a balun outer conductor disposed on the balun dielectric.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of and priority to U.S. Provisional Patent Application No. 61/806,605, filed on Mar. 29, 2013, and U.S. Provisional Patent Application No. 61/969,545, filed on Mar. 24, 2014, the entire contents of each of which are incorporated by reference herein for all purposes.

BACKGROUND

1. Technical Field

The present disclosure relates generally to microwave ablation applicators, and, more particularly, to reduced-size microwave ablation applicators and methods for manufacturing the same.

2. Discussion of Related Art

Electromagnetic fields can be used to heat and destroy tumor cells. Treatment may involve inserting ablation probes into tissues where cancerous tumors have been identified. Once the ablation probes are properly positioned, the ablation probes induce electromagnetic fields within the tissue surrounding the ablation probes.

In the treatment of diseases such as cancer, certain types of tumor cells have been found to denature at elevated temperatures that are slightly lower than temperatures normally injurious to healthy cells. Known treatment methods, such as hyperthermia therapy, heat diseased cells to temperatures above 41° C. while maintaining adjacent healthy cells below the temperature at which irreversible cell destruction occurs. These methods involve applying electromagnetic fields to heat or ablate tissue.

Devices utilizing electromagnetic fields have been developed for a variety of uses and applications. Typically, apparatuses for use in ablation procedures include a power generation source, e.g., a microwave generator that functions as an energy source, and a surgical instrument (e.g., microwave ablation probe having an antenna assembly) for directing energy to the target tissue. The generator and surgical instrument are typically operatively coupled by a cable assembly having a plurality of conductors for transmitting energy from the generator to the instrument, and for communicating control, feedback, and identification signals between the instrument and the generator.

There are several types of microwave probes in use, e.g., monopole, dipole, and helical, which may be used in tissue ablation applications. In monopole and dipole antenna assemblies, microwave energy generally radiates perpendicularly away from the axis of the conductor. Monopole antenna assemblies typically include a single, elongated conductor. A typical dipole antenna assembly includes two elongated conductors that are linearly-aligned and positioned end-to-end relative to one another with an electrical insulator placed therebetween. Helical antenna assemblies include helically-shaped conductor configurations of various dimensions, e.g., diameter and length. The main modes of operation of a helical antenna assembly are normal mode (broadside), in which the field radiated by the helix is maximum in a perpendicular plane to the helix axis, and axial mode (end fire), in which maximum radiation is along the helix axis.

The heating of tissue for thermal ablation is accomplished through a variety of approaches, including conduction of heat from an applied surface or element, ionic agitation by electrical current flowing from an electrode to a ground pad, optical wavelength absorption, or, in the case of microwave ablation, by dielectric relaxation of water molecules within an antenna electromagnetic field. The ablation zone can be broken down into two components: an active ablation zone and a passive ablation zone.

The active ablation zone is closest to the ablation device and encompasses the volume of tissue which is subjected to energy absorption high enough to assure thermal tissue destruction at a given application time in all but areas of very rapidly flowing fluids, such as around and within large blood vessels or airways. The active ablation zone size and shape is determined by ablation device design. The active ablation zone can therefore be used to produce predictable ablative effects over a given shape and volume of tissue.

The passive ablation zone surrounds the active zone and encompasses the volume of tissue which experiences a lower intensity of energy absorption. The tissue within the passive ablation zone may or may not experience tissue destruction at a given application time. Physiological cooling may counter heating from the lower level energy absorption and therefore not allow for sufficient heating to occur within the passive zone to kill tissue. Diseased or poorly perfused tissue within the passive zone may be more prone to heating than other tissues and may also be more susceptible to heat conduction from hotter areas within the ablation zone. The passive zone in these cases can result in unexpectedly large ablation zones. Due to these varying scenarios across space within a targeted physiology, relying on the passive zone to perform thermal ablation is challenging with unpredictable outcomes.

As electromagnetic fields can be induced at a distance by microwave probes, microwave ablation has the potential to create large active zones whose shapes can be determined and held constant by design. Furthermore, the shape and size can be determined through design to fit a specific medical application. By utilizing a predetermined active zone to create a predictable ablation zone, and not relying upon the indeterminate passive ablation zone, microwave ablation can provide a level of predictability and procedural relevance not possible with other ablative techniques.

The shape of the active zone about an antenna is determined by the frequency of operation, the geometry of the antenna, the materials of the antenna, and the medium surrounding the antenna. Operating an antenna in a medium of dynamically changing electrical properties, such as heating tissue, results in a changing shape of the electromagnetic field, and therefore a changing shape of the active zone. To maintain the shape of the active zone about a microwave antenna, the degree of influence on the electromagnetic field of the surrounding medium's electrical properties is reduced.

The size of the active zone about an antenna is determined by the amount of energy which can be delivered from the microwave generator to the antenna. With more energy delivered to the antenna, larger active zones can be generated. To maximize energy transfer from a microwave generator through waveguides and to a microwave antenna requires each system component to have the same impedance, or to be impedance matched. Whereas the impedance of the generator and waveguides are typically fixed, the impedance of a microwave antenna is determined by the frequency of operation, the geometry of the antenna, the materials of the antenna, and the medium surrounding the antenna. Operating an antenna in a medium of dynamically changing electrical properties, such as within heating tissue, results in a changing antenna impedance and varied energy delivery to the antenna, and, as a result, a changing size of the active zone. To maintain the size of the active zone about a microwave antenna, the degree of influence on the antenna impedance of the surrounding medium's electrical properties must be reduced.

In thermal ablation, the primary cause of active zone size and shape change is an elongation of the electromagnetic wave. Wavelength elongation occurs in heating tissue due to tissue dehydration. Dehydration reduces the dielectric constant, elongating the wavelength of microwave fields. Wavelength elongation is also encountered when a microwave device is used across various tissue types due to the varying dielectric constant between tissue types. For example, an electromagnetic wave is significantly longer in lung tissue than in liver tissue.

Wavelength elongation compromises the focus of microwave energy on the targeted tissue. With large volume ablation, a generally spherical active zone is preferable to focus the energy on generally spherical tissue targets. Wavelength elongation causes the electromagnetic field to stretch down along the length of the device toward the generator, resulting in a generally comet- or “hot-dog”-shaped active zone.

Wavelength elongation can be significantly reduced in medical microwave antennas by dielectrically buffering the antenna geometry with a material having an unchanging dielectric constant, as described in U.S. application Ser. Nos. 13/835,283 and 13/836,519, the disclosure of each of which are incorporated by reference herein. The material of unchanging dielectric constant surrounds the antenna, reducing the influence of the tissue electrical properties on antenna wavelength. By controlling wavelength elongation through dielectric buffering, the antenna impedance match and field shape can be maintained, enabling a large active ablation zone with a predetermined and robust shape.

By providing dielectric buffering with a circulated fluid, such as with saline or water, the high dielectric constants of these materials can be leveraged in the antenna geometry design, and furthermore the circulated fluid can be used to simultaneously cool the microwave components, including the coaxial feed line and antenna. Cooling of the microwave components also enables higher power handling of the components which can be used to deliver more energy to the antenna to create larger active zones.

As described above, the shape of the active zone about an antenna is determined, in part, by the geometry of the antenna. Ordinary ablation antennas do not utilize antenna geometry in combination with wavelength buffering to effectively control microwave field shape. These antennas do not create spherical active zone shapes nor are the active zones robust and unchanging across tissue types or during tissue heating. These antennas allow microwave energy to spread along the external conductor of the device from the device tip towards the generator. The spreading of microwave energy along the shaft results in comet- or “hot-dog”-shaped active zones.

Microwave antennas can be equipped with a choke or balun, a component of the antenna geometry that improves impedance matching and also can aid in focusing microwave energy into a predetermined shape. When combined with wavelength buffering, a balun or choke can effectively block the backwards propagation of electromagnetic waves along the external conductor toward the generator across various tissue types and during tissue heating, focusing the energy into a robust spherical active zone.

One implementation of a balun includes a balun dielectric that is disposed on the outer conductor of a coaxial cable and an outer balun conductor disposed on the balun dielectric. The balun creates a short section of coaxial waveguide arranged about the inner coaxial cable where the outer conductor of the coaxial cable is the inner conductor of the balun. The balun is disposed about the coaxial cable near the feed of the antenna and in one implementation has a length of λ/4 where λ is the wavelength of the electromagnetic wave within the balun. The balun outer conductor and inner conductor are shorted together at the proximal end to create a λ/4 short-circuited balun.

One way of describing the function of a λ/4 short-circuited balun is as follows: an electromagnetic wave propagates proximally along the radiating section of the antenna, enters the balun, reflects off of the short-circuited proximal end of the balun, propagates forward to the distal end of the balun, and exits the balun back onto the antenna radiating section. With this arrangement of balun length, when the electromagnetic wave reaches the distal end of the balun and travels back onto the antenna radiating section, the electromagnetic wave has accumulated a full λ of phase change. This is due to the λ/4 distance traveled forward within the balun, the λ/4 distance traveled backward within the balun and a λ/2 phase change which occurs with the reflection off of the short-circuited proximal end of the balun. The result is an electromagnetic wave which, rather than propagating along the external surface of the cable toward the generator, is a wave which is redirected back toward the distal tip of the antenna in coherent phase with the other waves on the antenna radiating section.

The balun, however, substantially increases the diameter of the microwave antenna as well as the needle through which the microwave antenna passes. The size of the needle may limit the uses for the microwave antenna in minimally-invasive procedures, especially when there are repeated treatments.

SUMMARY

In one aspect, the present disclosure features a microwave ablation applicator. The microwave ablation applicator includes a feed-line segment, a step-down segment, and a radiator base segment. The feed-line segment includes a first inner conductor, a first dielectric disposed on the outer surface of the first inner conductor and having a first face in a plane perpendicular to the longitudinal axis of the first inner conductor, and a first outer conductor disposed on the outer surface of the first dielectric. The step-down segment includes a second inner conductor, a second dielectric having a diameter less than the diameter of the first dielectric and disposed on the outer surface of the second inner conductor, and a second outer conductor disposed on the outer surface of the second dielectric.

The radiator base segment includes a third inner conductor, a third dielectric having a diameter less than the diameter of the first dielectric and disposed on the outer surface of the third inner conductor, a third outer conductor disposed on the outer surface of the proximal end of the third dielectric so as to form a feed gap at a distal end of the radiator base segment, a balun dielectric disposed on the outer surface of the third outer conductor, and a balun outer conductor disposed on the outer surface of the balun dielectric. The third outer conductor has a length that is less than the length of the third dielectric so as to leave a feed gap.

One or more of the feed-line segment, the step-down segment, and the radiator base segment may be rigid, semi-rigid, or flexible. The diameter of the second and third inner conductors may be equal to the diameter of the first inner conductor. The second and third inner conductors may be an extension of the first inner conductor. The outer diameter of the balun conductor may be equal to the outer diameter of the first outer conductor of the feed-line segment.

The step-down segment may taper from a first diameter at proximal end of the step-down segment to a second smaller diameter at a distal end of the step-down segment. The step-down segment may include multiple steps. A ferrule may be disposed at each of the steps of the step-down segment. The length of the step-down segment may be scaled by the dielectric constant of the second dielectric.

The feed-line segment, the step-down segment, and the radiator base segment are machined from a coaxial feed-line core may include an inner conductor and a dielectric insulator disposed around the inner conductor.

At least one of the first dielectric, the second dielectric, the third dielectric, and the balun dielectric may be dielectric tape. The second dielectric may be a foamed PTFE, low-density PTFE (LDPTFE), microporous PTFE, tape-wrapped PTFE, tape-wrapped and sintered PTFE, or PFA. The second outer conductor may be a silver-plated copper flat-wire braid, a solid-drawn copper tube, a conductive ink-coated PET heat shrink, a silver-coated PET heat shrink, or a silver-plated copper-clad steel braid.

In another aspect, the present disclosure features a microwave ablation applicator. The microwave ablation applicator includes a coaxial feed-line segment, an impedance step-down segment, a radiator base segment, and a coaxial balun disposed on the radiator base segment. The microwave ablation applicator also includes a radiating section attached to a distal end of the radiator base segment, and a dielectric buffering and cooling segment configured to receive the coaxial feed-line segment, the impedance step-down segment, the radiator base segment, and the radiating section.

One or more of the feed-line segment, the step-down segment, and the radiator base segment may be rigid, semi-rigid, or flexible. The outer diameter of the coaxial balun may be equal to or approximately equal to the outer diameter of the coaxial feed-line segment.

The dielectric buffering and cooling segment may include a first tube and a second tube disposed within the first tube. The second tube defines an outflow conduit between the inner surface of the first tube and the outer surface of the second tube, and defines an inflow conduit between the inner surface of the second tube and the outer surfaces of the coaxial cable and attached radiating section. The dielectric buffering and cooling segment may include a first tube defining inflow and outflow conduits for carrying cooling fluid.

BRIEF DESCRIPTION OF THE DRAWINGS

Objects and features of the presently disclosed energy-delivery devices with a fluid-cooled probe assembly and systems including the same will become apparent to those of ordinary skill in the art when descriptions of various embodiments thereof are read with reference to the accompanying drawings, of which:

FIG. 1 is a block diagram of a microwave ablation system in accordance with aspects of the present disclosure;

FIG. 2A is a perspective view of the microwave applicator of the microwave ablation system of FIG. 1 in accordance with aspects of the present disclosure;

FIG. 2B is a longitudinal, cross-sectional view of the microwave applicator of FIG. 2A;

FIG. 2C is a transverse, cross-sectional view of the microwave applicator of FIG. 2A in accordance with an aspect of the present disclosure;

FIG. 2D is a transverse, cross-sectional view of the microwave applicator of FIG. 2A in accordance with another aspect of the present disclosure;

FIG. 3A is a perspective view of a coaxial cable assembly after performing a step of a method of manufacturing the microwave applicator of FIGS. 2A-2D in accordance with aspects of the present disclosure;

FIG. 3B is a longitudinal, cross-sectional view of the coaxial cable assembly of FIG. 3A;

FIG. 4A is a perspective view of a coaxial cable assembly after performing another step of the method of manufacturing the microwave applicator of FIGS. 2A-2D;

FIG. 4B is a longitudinal, cross-sectional view of the coaxial cable assembly of FIG. 4A;

FIG. 5A is a perspective view of a coaxial cable assembly after performing still another step of the method of manufacturing the microwave applicator of FIGS. 2A-2D;

FIG. 5B is a longitudinal, cross-sectional view of the coaxial cable assembly of FIG. 5A;

FIG. 6A is a perspective view of a coaxial cable assembly after performing still another step of the method of manufacturing the microwave applicator of FIGS. 2A-2D;

FIG. 6B is a longitudinal, cross-sectional view of the coaxial cable assembly of FIG. 6A;

FIG. 7A is a perspective view of a coaxial cable assembly after performing still another step of the method of manufacturing the microwave applicator of FIGS. 2A-2D;

FIG. 7B is a longitudinal, cross-sectional view of the coaxial cable assembly of FIG. 7A;

FIG. 8A is a perspective view of a coaxial cable assembly after performing still another step of the method of manufacturing the microwave applicator of FIGS. 2A-2D;

FIG. 8B is a longitudinal, cross-sectional view of the coaxial cable assembly of FIG. 8A;

FIG. 9A is a perspective view of a coaxial cable assembly after performing still another step of the method of manufacturing the microwave applicator of FIGS. 2A-2D;

FIG. 9B is a longitudinal, cross-sectional view of the coaxial cable assembly of FIG. 9A;

FIG. 10A is a perspective view of a coaxial cable assembly after performing still another step of the method of manufacturing the microwave applicator of FIGS. 2A-2D;

FIG. 10B is a longitudinal, cross-sectional view of the coaxial cable assembly of FIG. 10A;

FIG. 11A is a perspective view of a partially completed coaxial cable assembly after performing a step of a method of manufacturing the partially completed coaxial cable assembly of FIGS. 7A and 7B in accordance with other aspects of the present disclosure;

FIG. 11B is a perspective view of a partially completed coaxial cable assembly after performing another step of the method of manufacturing the partially completed coaxial cable assembly of FIGS. 7A and 7B;

FIG. 11C is a perspective view of a partially completed coaxial cable assembly after performing still another step of the method of manufacturing the partially completed coaxial cable assembly of FIGS. 7A and 7B;

FIG. 12 is a perspective view of a partially completed coaxial cable assembly in accordance with still other aspects of the present disclosure;

FIG. 13A is a side view of a partially completed coaxial cable assembly after performing a step of a method of manufacturing the partially completed coaxial cable assembly in accordance with still other aspects of the present disclosure;

FIG. 13B is a side view of a partially completed coaxial cable assembly after performing another step of the method of manufacturing the partially completed coaxial cable assembly of FIG. 13A;

FIG. 14 is a flowchart of a method of manufacturing a microwave ablation applicator in accordance with an aspect of the present disclosure; and

FIG. 15 is a flowchart of a method of manufacturing a microwave ablation applicator in accordance with another aspect of the present disclosure.

DETAILED DESCRIPTION

The balun has the largest radial dimensions along the length of the microwave applicator. The present disclosure is generally directed to microwave ablation applicators and methods of manufacturing microwave ablation applicators having small radial dimensions. This is accomplished by reducing the radial dimensions of the landing on which the balun is built.

According to the present disclosure, the diameter of the antenna geometry may be reduced to be less than or equal to or approximately equal to the diameter of the coaxial feed-line. The miniaturization of the antenna geometry provides at least the following advantages: (1) it reduces the overall radial size of the microwave applicator without significantly compromising ablation performance or device strength; (2) it enables use of a larger coaxial cable feed-line, which reduces energy loss in the coaxial cable feed-line and thus increases energy delivery to the radiator; (3) it provides additional space within the microwave applicator without increasing overall radial size for various structures and features of the microwave applicator, such as the fluid channels, strengthening members, and centering features or sensors; and (4) it enables various manufacturing techniques, such as sliding the fully assembled microwave components into a multi-lumen catheter from one end, which would otherwise not be possible because of inconsistent radial dimensions between the microwave coaxial cable and the antenna.

With respect to endobronchial ablation, the miniaturization of the microwave applicator enables the technical feasibility (e.g., required tissue effect and appropriateness of the cooling) of a saline or water dielectric buffered and electrically choked (via the balun) microwave radiator at a 2.8 mm bronchoscope channel size. This further improves the tissue effect and cooling performance of the same application sized up to a 3.2 mm bronchoscope channel size device. Other intravascular, percutaneous, surgical, and laparoscopic applications where catheter size (French sizing) is of clinical significance are envisioned to benefit similarly. This may also provide space within the microwave applicator assemblies for thermocouple temperature sensors, which are described in U.S. application Ser. Nos. 13/836,519 and 13/924,277, the disclosure of each of which are incorporated by reference herein. Additionally, by maintaining a line-to-line dimension between the diameter of the feed-line coaxial segment and the diameter of the antenna geometry (including a balun), the microwave applicator assembly may be slid into a closed out (tipped) lumen from the proximal end, thus simplifying the manufacturing process. The manufacturing methods of the present disclosure may be used in the miniaturization and strengthening of ablation needles.

Embodiments of the microwave ablation systems and components are described with reference to the accompanying drawings. Like reference numerals may refer to similar or identical elements throughout the description of the figures. As shown in the drawings and as used in this description, the term “proximal” refers to that portion of the apparatus, or component of the apparatus, closer to the user and the term “distal” refers to that portion of the apparatus, or a component of the apparatus, farther from the user.

This description may use the phrases “in an embodiment,” “in embodiments,” “in some embodiments,” or “in other embodiments,” which may each refer to one or more of the same or different embodiments in accordance with the present disclosure.

As it is used in this description, “microwave” generally refers to electromagnetic waves in the frequency range of 300 megahertz (MHz) (3×108 cycles/second) to 300 gigahertz (GHz) (3×1011 cycles/second). As it is used in this description, “ablation procedure” generally refers to any ablation procedure, such as, for example, microwave ablation, radiofrequency (RF) ablation, or microwave or RF ablation-assisted resection. As it is used in this description, “transmission line” generally refers to any transmission medium that can be used for the propagation of signals from one point to another. As it is used in this description, “fluid” generally refers to a liquid, a gas, or both.

FIG. 1 is a block diagram of a microwave tissue treatment system 10 in accordance with aspects of the present disclosure. The microwave tissue treatment system 10 includes a microwave tissue treatment device 20 having a microwave applicator or antenna assembly 100 connected to a microwave generator 40 through a feedline 60. The microwave tissue treatment device 20 may include one or more pumps 80, e.g., a peristaltic pump or the like, for circulating a cooling or heat dissipative fluid through the microwave applicator or antenna assembly 100 via an inflow fluid conduit 182 and an outflow fluid conduit 184 of a cooling system 180. The mechanical functionality of the pump in driving fluid through the system may be substituted by driving the fluid with pressurized and regulated reservoirs.

The feedline 60 may range in length from about 7 feet to about 10 feet, but may be either substantially longer or shorter if required in a particular application. The feedline 60 transfers microwave energy to microwave tissue treatment device 20. The feedline 60 includes a coaxial cable having an inner conductor, an outer conductor, and a dielectric interposed between the inner and outer conductors. The dielectric electrically separates and/or isolates the inner conductor from the outer conductor. The feedline 60 may further include any sleeve, tube, jacket, or the like formed of any conductive or non-conductive material. The feedline 60 may be separable from, and connectable to, the antenna assembly 100 or the microwave tissue treatment device 20.

The inner and outer conductors are each formed, at least in part, of a conductive material or metal, such as stainless steel, copper, or gold. In certain embodiments, the inner and outer conductors of feedline 60 may include a conductive or non-conductive substrate that is plated or coated with a suitable conductive material. The dielectric may be formed of a material having a dielectric value and tangential loss constant of sufficient value to electrically separate and/or isolate the respective inner and outer conductors from one another, including but not being limited to, expanded foam polytetrafluoroethylene (PTFE), polymide, silicon dioxide, or fluoropolymer. The dielectric may be formed of any non-conductive material capable of maintaining the desired impedance value and electrical configuration between the respective inner and outer conductors. In addition, the dielectric may be formed from a combination of dielectric materials.

The antenna assembly 100 of the microwave tissue treatment system 10 includes a coaxial feed-line segment 112, a impedance step-down segment 114, a radiator base segment 116 on which a choke or coaxial balun 118 is disposed, a distal radiating section 120, and a dielectric buffering and cooling structure 122.

The proximal portion of the antenna assembly 100 may include a connecting hub 140. The connecting hub 140 defines a conduit configured and dimensioned to receive a distal end of the feedline 60, additional conduits configured and dimensioned to receive the inflow conduit 182 and the outflow conduit 184 of the cooling system 180, and one or more apertures formed in an internal surface of the connecting hub 160 that are configured and dimensioned to receive the inflow conduit 182 and the outflow conduit 184, respectively. Connecting hub 160 may be formed of any suitable material including, but not limited to, polymeric materials. Although not explicitly shown, the hub may also include conduits configured and dimensioned to receive sensors, including but not limited to thermocouples or impedance monitoring electrodes.

As described above, the microwave ablation applicator of the present disclosure minimizes the radial dimension of a coaxial-fed microwave ablation applicator. Specifically the radial dimension of the balun, which is largest radial dimension along the microwave ablation radiator, is minimized. This may be accomplished by reducing the dimension of the landing on which the balun is built.

As shown in FIG. 1, the microwave ablation applicator includes six segments or structures: (1) the coaxial feed-line segment 112 of the coaxial cable, (2) the impedance step-down segment 114 of the coaxial cable, (3) the radiator base segment 116 of the coaxial cable, (4) the coaxial balun 118, (5) the distal radiating section 120, and (6) the dielectric buffering and cooling structure 122, which includes conduits for carrying dielectric buffering and cooling fluid through the microwave applicator to dielectrically buffer and cool at least the radiator base segment 116 and distal radiating section 120. The construction of and the materials used for each of these segments or structures will now be described.

FIGS. 2A-2D show the microwave applicator inserted into the dielectric buffering and cooling structure 122. The coaxial feed-line segment 112 (FIG. 1) may be constructed of a coaxial cable of any variety, including a rigid, semi-rigid, or flexible coaxial cable. The impedance of the waveguide formed by the coaxial cable may be 50 ohms, but may range from 20 ohms to 150 ohms. An inner conductor 212 of the coaxial feed-line segment 112 is surrounded by a dielectric insulator 214, which, in turn, is partially or fully covered by an outer conductor 216 (also referred to as a shield). For a 150 Watt, 2450 MHz design, for example, the inner conductor 212 may have a diameter of 0.02870 cm, the dielectric insulator 214 may have a diameter of 0.09144 cm, and the outer conductor 216 may have a diameter of 0.1054 cm.

The inner conductor 212 may be a silver-plated solid copper wire. The dielectric insulator 214 may be an extruded polytetrafluoroethylene (PTFE) dielectric insulator, wrapped PTFE, foamed PTFE, or perfluoroalkoxy (PFA). The outer conductor 216 may be a silver-plated copper wire braid constructed from either flat or round braid wire. A jacket (not shown) for environmental and mechanical robustness may be applied onto or melted into the braided shield. The jacket may be a heat shrink material, such as polyethylene terephthalate (PET) or fluorinated ethylene propylene (FEP), or an extruded thermoplastic.

The impedance step-down segment 114 may include an inner conductor 222 that is the same as the inner conductor 212 of the coaxial feed-line segment 112. Thus, the inner conductor 222 may be unchanged and seamless between the coaxial feed-line segment 112 and the impedance step-down segment 114 to simplify manufacture of the microwave applicator and improve electrical performance. In other words, the inner conductor 222 may be an extension of the inner conductor 212. In embodiments, the radial dimension of the inner conductor 222 may be reduced. The difference between the coaxial feed-line segment 112 and the impedance step-down segment 114 is that the outer radial dimension of the impedance step-down segment 114 is reduced according to the calculations described below.

The length of the impedance step-down segment 114 may be optimized for electrical performance at one quarter of the wavelength of the frequency of operation. The length of the impedance step-down segment 114 may be scaled by the dielectric constant of the impedance step-down segment's dielectric insulator 224. For example, the length of the impedance step-down segment 114 may be 2.1 cm for an operation frequency of 2450 MHz. In other embodiments, the length of the impedance step-down segment 114 may deviate from a quarter wavelength. For example, the length of the impedance step-down segment 114 may be 5.6 cm for an operation frequency of 915 MHz and 0.9 cm for 5800 MHz. In yet other embodiments, the impedance step-down segment 114 may be stepped down using a variety of approaches including a taper step down (described in more detail below), a multiple segment step down (also described in more detail below), or an exponential tapering.

The impedance step-down segment 114 may be constructed from the same materials as the coaxial feed-line segment 112, or the impedance step-down segment 114 may use a different combination of materials than the coaxial feed-line segment 112. The dielectric insulator 224 may be a foamed PTFE, such as low-density PTFE (LDPTFE) or microporous PTFE, tape-wrapped PTFE, tape-wrapped and sintered PTFE, or PFA. The outer conductor 226 may be a silver-plated copper flat wire braid, a solid-drawn copper tube, a conductive ink-coated PET heat shrink (e.g., silver ink-coated PET heat shrink), or a silver-plated copper-clad steel braid.

The radiator base segment 116 may include an inner conductor 232 that is unchanged and seamless with the inner conductor 222 of the impedance step-down segment 114 and the inner conductor 212 of the coaxial feed-line segment 112, which would simplify manufacture of the radiator base segment 116 and would improve electrical performance. If the inner conductor 232 of the radiator base segment 116 were to change with the radiator base segment 116, its radial dimension may be reduced. A difference between the radiator base segment 116 and the impedance step-down segment 114 is that the radial dimension of the radiator base segment's dielectric insulator 234 is reduced according to the calculations described below.

The far distal end of the outer conductor or shield 236 of the radiator base segment 116 is removed to create the feed gap 238, which allows microwave fields to propagate onto the distal radiating section 120 from the coaxial waveguide. The length of the radiator base segment 116 is approximately equal to the sum of the lengths of the coaxial balun 118, the feed gap 238, and the proximal radiating arm, which is the length between the coaxial balun 118 and the feed gap 238. For example, for an operating frequency of 2450 MHz, the coaxial balun 118 may have a length of 2 cm, the proximal radiating arm may have a length of 1 cm, and the feed gap 238 may have a length of 0.3 cm.

The radiator base segment 116 may be constructed from the same materials as or different materials from the coaxial feed-line segment 112 and/or the impedance step-down segment 114. The dielectric insulator 234 of the radiator base segment 116 may be a low-density PTFE (e.g., a foamed PTFE), a tape-wrapped PTFE, a tape-wrapped and sintered PTFE, or a PFA. The outer conductor 236 may be a silver-plated copper flat-wire braid, a solid-drawn copper tube, a silver ink-coated PET heat shrink, or a silver-plated copper-clad steel braid.

The coaxial balun 118 is assembled on top of the radiator base segment as shown in FIGS. 2B-2D. The coaxial balun 118 is composed of a balun dielectric insulator 244 and a balun outer conductor 246 (also referred to as a balun shield).

The overall outer diameter of the coaxial balun 118 may be set equal to or approximately equal to the overall outer diameter of the coaxial feed-line segment 112, such that the largest overall radial dimension of the device is not increased by the coaxial balun 118. For example, the overall outer diameter of the coaxial balun 118 may be 0.105 cm. This equality sets the initial conditions of the design calculations described below. The length of the coaxial balun 118 is equal to one quarter of the wavelength of the frequency of operation, which is scaled by the dielectric constant of the balun dielectric insulator 244. For example, the length of the coaxial balun 118 may be 2.0 cm in length for operation at 2450 MHz. The balun dielectric insulator 244 may extend beyond the distal end of the coaxial balun outer conductor or shield 246, as shown in FIG. 2B (insulator extension 248), which enhances the effectiveness of the coaxial balun 118 across a variety of physiological conditions. For example, the length of the extended balun dielectric insulator 244 may be 0.3 cm.

The coaxial balun 118 may be constructed from the same materials as the coaxial feed-line segment 112, or may vary from the specific materials of the coaxial feed-line segment 112. For example, the coaxial dielectric insulator 244 may be a foamed PTFE (LDPTFE), tape-wrapped PTFE, tape-wrapped and sintered PTFE, or PFA. The balun outer conductor 246 may be a silver-plated copper flat-wire braid, a solid-drawn copper tube, a silver ink-coated PET heat shrink, or a silver-plated copper-clad steel braid.

The distal radiating section 120 is an elongated conductor which is soldered, crimped, or welded onto the distal end of the inner conductor 232 of the radiator base segment 116. The shape of the distal radiating section 120 may be a cylinder. Alternatively, the distal radiating section 120 may be composed of several cylinders of varying diameter, such as a barbell or pin with a widened base. Additional heat-sinking features, such as burs and fins, may be added to the distal radiating section 120 to increase the radiating effectiveness of the microwave applicator. These features, such as the barbell mentioned above, may also help to center the radiator within the dielectric buffering and cooling structure 122.

The length of the distal radiating section 120 may be designed for approximately one quarter wavelength at the frequency of operation. For example, the length of the distal radiating section 120 may be approximately 1 cm for an operation frequency of 2450 MHz. Alternatively, the distal radiating section 120 may be reduced or lengthened to match the line impedance of the radiator base segment 116 to the overall antenna impedance. Increasing or decreasing the length of the distal radiating section 120 proportionally reduces or increases, respectively, the length of the proximal radiating arm, maintaining the overall length of the antenna at half wavelength resonance at the frequency of operation. For example, the total length of the distal radiating section 120, including the feed gap 238, may be approximately 2.3 cm for operation at 2450 MHz.

The distal radiating section 120 may be gold-plated brass, silver-plated copper, or any other composite of materials having high surface conductivity, such as a polymer rod with conductive coating. The distal radiating section 120 may also be created by extending the radiator base segment's dielectric insulator 234 and inner conductor 232 an appropriate length and covering with a conductive surface, such as electroplating, conductive ink, wrapped foil, or braided wire.

The dielectric buffering and cooling structure 122 includes a mechanical support for the device, circulated cooling fluid, such as gas or liquid, and chambers to enable the circulation of the fluid, such as concentric inflow and outflow tubes 202 and 203 forming fluid paths 208 and 206, respectively, or multi-lumen thermo-plastic extrusion, e.g., lumens 204-207. The dielectric buffering of the antenna from the surrounding tissue environment is provided by the circulated liquid extending over the length of the radiating section. Alternatively, the cooling lumens and fluids may terminate proximal to the distal radiating section 120 and high dielectric solid material may be disposed distally over the radiating section of the microwave applicator to dielectrically buffer the antenna and provide mechanical stiffness.

The dielectric buffering and cooling structure 122 may be composed of various thermoplastics and may be manufactured according to a multi-lumen extrusion approach. The dielectric buffering and cooling structure 122 may include an outflow tube 203 composed of fiber glass and an inflow tube 202 composed of polyimide or PET extrusion and may be manufactured according to a concentric approach, in which materials are layered upon each other. The inflow tube 202 and the outflow tube 203 may alternatively be composed of a Kevlar braid thermoplastic composite. The cooling fluid may be water, saline, or any common water-based liquid. The high dielectric solid material may be a ceramic material, such as Yttria Tetragonal Zirconia Polycrystal (YTZP).

In embodiments, the microwave ablation applicator may be designed by first optimizing the step-down dielectric design. One example approach to determining the dimensions of the feed-line segment, the step-down segment, and the radiator base segment of the microwave ablation applicator is to constrain the outer diameter of the balun to the outer diameter of the feed-line so that the outer diameter of the microwave ablation applicator assembly is no larger than the outer diameter of the feed-line. The lengths and diameters of each segment may also be designed to achieve low insertion loss through the antenna feed gap at a frequency of operation. The frequency of operation may be a bandwidth of operation, such as from 2400 MHz to 2500 MHz. After determining the dimensions of the segments of the microwave ablation applicator, the distal radiating section, balun, and dielectric buffering and cooling structures are added to the design, and the dimensions of the segments are then further optimized to achieve a controlled energy pattern and high energy-to-tissue efficiency.

The dimensions of the segments of the microwave ablation probe may be determined by starting with the target balun radial dimensions, which may be chosen to be approximately equal to or smaller than the radial dimensions of the coaxial feed-line segment 112. Next, the dimensions of the radiator base segment 116 are determined, and then the dimensions of the step-down segment 114 are determined using a quarter-wave matching equation. The quarter-wave matching equation matches the impedance change between the larger coaxial feed-line segment 112 and the smaller radiator base segment 116. This method of determining the dimensions of the segments of the microwave ablation probe is illustrated by the following example.

First, the dimensions of the coaxial feed-line segment 112 are calculated. The diameter of the inner conductor 212 (IC₁) of the coaxial feed-line segment 112 is calculated as follows:

$\begin{matrix} {{{IC}_{1} = e^{{{- Z_{feed}}{\sqrt{ɛ_{r\; 1}} \cdot 2}\;\pi\sqrt{ɛ_{0/\mu_{0}}}} + {\log{({OD}_{{dielectric}\; 1})}}}},} & (1) \end{matrix}$ where Z_(feed) is the impedance of the coaxial feed-line segment 112, ∈_(r1) is the dielectric constant of the dielectric insulator 214 of the coaxial feed-line segment 112, ∈₀ is the permittivity of free space or vacuum, μ₀ is the permeability of vacuum, and OD_(delectric1) is the outer diameter of the dielectric insulator 214. Equation (1) is derived from the equation for the impedance of a coaxial cable. The total or outer diameter of the coaxial feed-line segment 112 (OD_(cable) ₁ ) is calculated as follows: OD_(cable) ₁ =OD_(dielectric) ₁ +ODadd_(braid) ₁ +ODadd_(jacket) ₁   (2) where ODadd_(braid) ₁ is the diameter addition from the outer conductor 216 and ODadd_(jacket) ₁ is the diameter addition from the jacket of the coaxial feed-line segment 112.

The dimensions of the coaxial balun 118 may be calculated by setting the outer diameter of the balun structure or choke (OD_(choke)) equal to the outer diameter of the coaxial feed-line segment 112 (OD_(cabie) ₁ ), which may be expressed by the following equation: OD_(choke)=OD_(cable) ₁   (3) In other embodiments, the outer diameter of the choke (OD_(choke)) may be set less than the outer diameter of the coaxial feed-line segment 112 (OD_(cable) ₁ ). The outer diameter of the choke's dielectric insulator 244 (OD_(choke dielectric)) is then determined from the following equation: OD_(choke dielectric)=OD_(choke)−ODadd_(choke jacket)−ODadd_(choke braid),  (4) where ODadd_(choke jacket) is the diameter addition from the choke's jacket and ODadd_(choke braid) is the diameter addition from the outer conductor 246 of the choke or coaxial balun 118. The inner diameter of the balun insulator 244 (ID_(choke dielectric)) is then determined from the following equation: ID_(choke dielectric)=OD_(choke dielectric)−OD_(choke dielectric)  (5)

Next, the dimensions of the radiator base segment 116 are calculated. First, the diameter of the inner conductor 232 of the radiator base segment 116 (IC₃) is set equal to the diameter of the inner conductor 212 (IC₁) of the coaxial feed-line segment 112 (IC₁), that is: IC₃=IC₁  (6) The outer diameter of the radiator base segment's insulator 234 (OD_(dielectric) ₃ ) is then calculated as follows: OD_(dielectric) ₃ =ID_(choke dielectric)−ODadd_(braid) ₃ −ODadd_(jacket) ₃   (7) where ODadd_(braid) ₃ is the diameter addition from the outer conductor 246 of the choke or coaxial balun 118 and ODadd_(jacket) ₃ is the diameter addition from the choke's jacket.

Then, the inner diameter of the radiator base segment's insulator 234 (ID_(dieletric) ₃ ) is set equal to the diameter of the inner conductor 232 of the radiator base segment 116 (IC₃), that is: ID_(dielectric) ₃ =IC₃  (8) The outer diameter of the cable for the radiator base segment 234 (OD_(cable) ₃ ) is then calculated according to the following equation: OD_(cable) ₃ =OD_(dielectric) ₃ +ODadd_(braid) ₃ +ODadd_(jacket) ₃   (9)

Next, the step-down impedance is calculated by first calculating the feed-line and radiator base segment impedances. The impedance of the coaxial feed-line segment 112 is given by the equation:

$\begin{matrix} {Z_{{cable}_{1}} = \frac{\log\left( {{OD}_{{dielectric}_{1}}/{IC}_{1}} \right)}{2\;\pi\sqrt{\mu_{0}/\left( {ɛ_{0} \cdot ɛ_{r_{1}}} \right)}}} & (10) \end{matrix}$

The impedance of the radiator base segment 116 is given by the equation:

$\begin{matrix} {Z_{{cable}_{3}} = \frac{\log\left( {{OD}_{{dielectric}_{3}}/{IC}_{3}} \right)}{2\;\pi\sqrt{\mu_{0}/\left( {ɛ_{0} \cdot ɛ_{r_{3}}} \right)}}} & (11) \end{matrix}$ where ∈_(r3) is the dielectric constant of the dielectric insulator 234 of the radiator base segment 116.

The impedance of the step-down segment 114 may be calculated using the quarter wave impedance transformer approach by taking the square root of the product of the impedances of the coaxial feed-line segment 112 (9) and radiator base segment 116 (10) as shown in the following equation: Z _(cable) ₂ =√{square root over (Z _(cable) ₁ ·Z _(cable) ₃ )}  (12)

The dimensions of the step-down segment 114 may then be calculated as follows. The diameter of the inner conductor 222 of the step-down segment 114 is set equal to the diameter of the inner conductor 212 of the coaxial feed-line segment 112, that is: IC₂=IC₁  (13) Then, the outer diameter of the step-down segment's insulator 224 (OD_(dielectric) ₂ ) is calculated using the following equation:

$\begin{matrix} {{{OD}_{{{dielectric}\;}_{2}} = e^{{Z_{{cable}_{2}}{\sqrt{ɛ_{r_{2}}} \cdot 2}\;\pi\sqrt{ɛ_{0}/\mu_{0}}} + {\log{({IC}_{2})}}}},} & (14) \end{matrix}$ where ∈_(r) ₂ is the dielectric constant of the dielectric insulator 224 of the step-down segment 114. Equation (14) is derived from the impedance of the coaxial cable equation.

The inner diameter of the step-down segment's insulator 224 is given by the following equation: ID_(dielectric) ₃ =IC₃  (15) Then, the total outer diameter of the step-down segment 114 is calculated according to the equation: OD_(cable) ₂ =OD_(dielectric) ₂ +ODadd_(braid) ₁ +ODadd_(jacket) ₁   (16)

Using the dimensions determined from the design of the microwave ablation applicator described above, the microwave ablation applicator may be manufactured according to a variety of methods, examples of which are described below. For example, FIGS. 3A-10B illustrate a method for manufacturing the microwave applicator by stacking dielectric cylinders. As shown in FIGS. 3A and 3B, the manufacturing method begins with a coaxial feed-line segment having a first inner conductor 212, a first insulator 214, and an outer conductor 216; an impedance step-down segment 114 having a second inner conductor 222; and a radiator base segment 232 having a third inner conductor. This configuration may be formed by stripping the insulator and outer conductor from a distal portion of a coaxial cable.

Next, as shown in FIGS. 4A and 4B, a second insulator 224 having a diameter less than the diameter of the first insulator 214 is disposed around the second inner conductor 222 of the impedance step-down segment 114. This may be accomplished by sliding a cylindrical insulator onto the second inner conductor 222. Next, as shown in FIGS. 5A and 5B, an outer conductor 226 is disposed on the surface of the second insulator 224 of the impedance step-down segment 114.

In the next step of the manufacturing method, a third insulator 234 having a diameter less than the diameters of the first insulator 214 and the second insulator 224 is disposed around the third inner conductor 232 of the radiator base segment 116, as shown in FIGS. 6A and 6B. This may be also accomplished by sliding a cylindrical insulator onto the third inner conductor 232. Next, as shown in FIGS. 7A and 7B, an outer conductor 236 is disposed on the surface of a proximal portion of the third insulator 234 of the radiator base segment 116 leaving a distal portion of the third insulator 234 exposed to form a feed gap 238.

Next, a balun insulator 244 is disposed around the proximal end of the third outer conductor 236 of the radiator base segment 116, as shown in FIGS. 8A and 8B. This may be accomplished by sliding a cylindrical balun insulator onto the proximal end of the third outer conductor 236. Next, a balun outer conductor 246 is disposed on the surface of a proximal portion of the balun insulator 244 leaving a distal portion of the balun insulator 244 exposed to form a feed gap 238, as shown in FIGS. 9A and 9B. Lastly, the distal radiating section 120 is attached to the distal end of the third inner conductor 232, as shown in FIGS. 10A and 10B. As shown in the figures, an electrical connection is made between the various outer conductors 216, 226, and 236 where they exist in proximity to one another, such as at the transitions between coaxial segments. The balun outer conductor 246 is also electrically connected at its proximal end to the coaxial outer conductors 226 and 236.

Another embodiment of the method of manufacturing a microwave applicator according to the stacking approach starts with a feed-line core (e.g., the feed-line core of FIG. 11A), including an inner conductor (e.g., inner conductor 1102) and insulator (e.g., insulator 1104). The insulator is stripped off of the distal length of the feed-line core equal to the length of the step-down segment and the radiator base segment. An insulator cylinder for the step-down segment is slid onto the inner conductor so that it is flush with the stripped feed-line insulator face. Next, an insulator cylinder of the radiator base segment is slid onto the inner conductor so that it is flush with the step-down insulator distal face. Hydrophilic gel, grease, or fluid may be applied between the faces of dielectric and onto the inner conductor to assist in slide assembly and to resist highly pressurized cooling fluid ingress into these spaces.

The braiding of the outer conductor over the stacked assembly would then be performed along the entire length of the stacked assembly. Conductive or dielectric ferrules may be added to the step faces of the insulator to improve the transition of the braid from one segment to the next, as shown in FIG. 13B. The braid is stripped back to form a feed gap on the radiator base segment. The balun insulator is then slid over, heat shrinked onto, or wrapped around the radiator base segment. A braid is placed over the balun insulator such that the proximal end of braid electrically shorts to the radiator base segment. A ferrule (choke short conductor) may be added to the proximal face of the balun dielectric to simplify electrical and mechanical termination of the balun braid. The balun braid is stripped back to expose the balun extended dielectric. Then, the distal radiating section 120 is attached to the radiator base segment's inner conductor 232. The distal radiating section 120 may be soldered, crimped, or welded to the radiator base segment's inner conductor 232. The antenna assembly is then slid into a cooling and dielectric buffering structure.

FIG. 14 illustrates a method of manufacturing a microwave applicator subassembly according to the stacking approach. After the method starts in step 1401, a first dielectric and a first outer conductor is stripped from a distal portion of a coaxial cable to expose a first inner conductor, in step 1402. In step 1404, a second dielectric is slid onto the inner conductor so that a proximal transverse face of the second dielectric abuts a distal transverse face of the first dielectric. In step 1406, a second outer conductor is applied to the surface of the second dielectric. In step 1408, a third dielectric is slid onto a remaining distal portion of the inner conductor so that a proximal transverse face of the third dielectric abuts a distal transverse face of the second dielectric. In step 1410, a third outer conductor is applied to the surface of the third dielectric so as to leave a distal portion of the third dielectric exposed.

In step 1412, a balun dielectric is slid onto a proximal portion of the third dielectric so as to leave a distal portion of the third outer conductor exposed. In step 1414, a balun outer conductor is applied to the surface of the balun dielectric so as to leave a distal portion of the balun dielectric exposed. Then, before the method ends in step 1417, the radiating section is attached to the distal end of the inner conductor, in step 1416.

Alternatively, the microwave applicator may be manufactured by machining a coaxial feed-line core, which is illustrated in FIG. 11A. Starting with the coaxial feed-line core, which includes an inner conductor 1102 and insulator 1104, the insulator 1104 is machined down to a profile consistent with the geometries of the step-down segment 114 and radiator base segment 116, as shown in FIG. 11B. Then, as shown in FIG. 11C, a first outer conductor 216 is applied to the surface of the dielectric of the coaxial feed-line segment, a second outer conductor 226 is applied to the surface of the dielectric of the impedance step-down segment, and a third outer conductor 236 is applied to the surface of the proximal portion of the dielectric of the radiator base segment to leave the distal end of the dielectric exposed to form the feed gap 238. In other embodiments, an outer conductor is braided over the length of machined core profile. The braid is stripped back to form the feed gap 238.

The balun insulator 244 is then slid onto, heat shrinked onto, or wrapped over the radiator base segment 116. The balun conductor 246 is braided over the balun insulator 244 such that the proximal end of the balun conductor 246 electrically shorts to the radiator base segment 116. The balun braid is stripped back to expose a distal portion of the balun insulator 244. The distal radiating section 120 is then soldered, crimped, or welded onto the radiator base segment's inner conductor 232. Finally, the antenna assembly is slid into the cooling and dielectric buffering and cooling structure 122.

In some embodiments, the coaxial feed-line core of FIG. 11A may be machined down to the geometry shown in FIG. 12, which tapers the diameter of the impedance step-down segment 1214. As shown in FIG. 12, the diameter of the impedance step-down segment 1214 tapers from a first diameter at a proximal end of the impedance step-down segment 1214 to a second smaller diameter at a distal end of the impedance step-down segment 1214.

In other embodiments, the coaxial feed-line core of FIG. 11A may be machined down to the geometry shown in FIG. 13A. As shown, the impedance step-down segment 1314 includes multiple steps 1351, 1353, and 1355 having different diameters. Conductive or dielectric ferrules 1362, 1364, and 1366 may be disposed at the respective step faces 1352, 1354, and 1356 of respective steps 1351, 1353, and 1355 to provide a smooth transition between steps so that an outer conductor may be easily applied to the length of the microwave applicator.

FIG. 15 illustrates a method 1500 of manufacturing a microwave applicator subassembly according to the machining approach. After the method starts in step 1501, dielectric material is removed from a middle portion of a coaxial feed-line core to form a step-down segment, in step 1502. In step 1504, dielectric material is removed from a distal portion of the coaxial feed-line core to form a radiator base segment. In step 1506, an outer conductor is applied over the length of the coaxial feed-line core. In step 1508, the outer conductor is removed from the distal end of the radiator base segment to form a feed gap. In step 1510, a balun dielectric onto a proximal portion of the radiator base segment. In step 1512, a balun outer conductor is applied to the surface of the balun dielectric. In step 1514, a balun outer conductor is removed from a distal portion of the balun dielectric. In step 1516, a radiating section is welded to the distal end of the inner conductor of the coaxial feed-line core. Then, in step 1517, the method of FIG. 15 ends.

As another alternative, the microwave applicator may be manufactured through selective removal of tape-wrapped dielectric. The profile of a tape-wrapped dielectric core could be made to match the described step-down segment profile by removing one or more layers of the tape along the appropriate length of the core. The remaining process would match the machining approach.

Although embodiments have been described in detail with reference to the accompanying drawings for the purpose of illustration and description, it is to be understood that the inventive processes and apparatus are not to be construed as limited thereby. It will be apparent to those of ordinary skill in the art that various modifications to the foregoing embodiments may be made without departing from the scope of the disclosure. 

What is claimed is:
 1. A microwave ablation applicator comprising: a feed-line segment including a first inner conductor, a first dielectric disposed on an outer surface of the first inner conductor and having a first face in a plane perpendicular to a longitudinal axis of the first inner conductor, and a first outer conductor disposed on the outer surface of the first dielectric; a step-down segment adjacent to a distal end portion of the feed-line segment and including a second inner conductor, a second dielectric having a diameter less than a diameter of the first dielectric and disposed on the outer surface of the second inner conductor, and a second outer conductor disposed on the outer surface of the second dielectric; and a radiator base segment adjacent to a distal end portion of the step-down segment and including a third inner conductor, a third dielectric having a diameter less than the diameter of the first dielectric and disposed on the outer surface of the third inner conductor, a third outer conductor disposed on the outer surface of a proximal end portion of the third dielectric so as to form a feed gap at a distal end portion of the radiator base segment, a balun dielectric disposed on the outer surface of the third outer conductor, and a balun outer conductor disposed on the outer surface of the balun dielectric, the third outer conductor having a length that is less than a length of the third dielectric so as to form the feed gap, wherein a length of the step-down segment is based on a wavelength of an operation frequency of the microwave ablation applicator, wherein the balun outer conductor is directly connected to a proximal end portion of the third outer conductor, wherein the step-down segment includes multiple steps, and wherein a ferrule is disposed at each of the steps of the step-down segment.
 2. The microwave ablation applicator according to claim 1, wherein one or more of the feed-line segment, the step-down segment, and the radiator base segment is rigid, semi-rigid, or flexible.
 3. The microwave ablation applicator according to claim 1, wherein the diameter of the second and third inner conductors are equal to the diameter of the first inner conductor.
 4. The microwave ablation applicator according to claim 1, wherein the second and third inner conductors are an extension of the first inner conductor.
 5. The microwave ablation applicator according to claim 1, wherein the outer diameter of the balun outer conductor is equal to or approximately equal to the outer diameter of the first outer conductor of the feed-line segment.
 6. The microwave ablation applicator according to claim 1, wherein the step-down segment tapers from a first diameter at a proximal portion of the step-down segment to a second smaller diameter at a distal portion of the step-down segment.
 7. The microwave ablation applicator according to claim 1, wherein the length of the step-down segment is scaled by the dielectric constant of the second dielectric.
 8. The microwave ablation applicator according to claim 1, wherein the feed-line segment, the step-down segment, and the radiator base segment are machined from a coaxial feed-line core including an inner conductor and a dielectric insulator disposed around the inner conductor.
 9. The microwave ablation applicator according to claim 1, wherein at least one of the first dielectric, the second dielectric, the third dielectric, and the balun dielectric is dielectric tape.
 10. The microwave ablation applicator according to claim 1, wherein the second dielectric is a foamed PTFE, low-density PTFE (LDPTFE), microporous PTFE, tape-wrapped PTFE, tape-wrapped and sintered PTFE, or PFA.
 11. The microwave ablation applicator according to claim 1, wherein the second outer conductor is a silver-plated copper flat-wire braid, a solid-drawn copper tube, a conductive ink-coated PET heat shrink, a silver-coated PET heat shrink, or a silver-plated copper-clad steel braid. 